I have a MacBook Pro that I had my iPhoto library on until a few months ago.
I wanted to free up space and move my 50+GB library to a Seagate external drive. That worked fine. Over the past month, I had my college aged daughter work on deleting bad photos by plugging in the external drive to my MacBook Pro or our iMac. Now somehow the whole library is on my MacBook, and on my external drive, and on my iMac. She said to start out with it asked her what library to open. (I named it Correct and set the computers actual libraries to DoNotUse). Now iPhoto Correct Library exists in all 3 places, and the actual GB size differs on all 3.
I am scared to delete and delete the wrong one that might be just thumbnails or some other unusable files. (Not really sure on what iPhoto completely saves).
Was there something in the settings that caused it to duplicate the libraries?

My daughter is a Video Communications student that has been migrating from PC to Mac. I slowly converted her earlier this year to an iPad, an iPhone, and then bought her the virtually unused used iMac as her college video production lab is all Mac-based(And an amazing one at that). I am trying to preach the virtues of Mac and now their software is making me pull out our hair!

Whatever happened she did it inadvertently and we can't seem to Google an answer or a solution. But I need to fix it! I have a couple thousand digitally converted photos from old family slides and don't want to import them and start working with organizing them until I get rid of the bad libraries. But since this is my entire families and my moms entire families photo library I don't want to make a mistake on the deletion.

First of all... my #1 priority is to make sure that you are not inadvertently deleting any photos by mistake. Hence... make sure that 100% of the libraries are stored someplace safely... and backed up. The bigger problem is if you lose photos. Having duplicates or "crap photos" is OK, and trivial to fix.

Secondly... I would recommend that you ditch iPhoto and switch to Aperture 3 (A3)... and spend some time learning it if you want to really simplify your "digital life". The program is so good... that it is reason enough to switch to a Mac.

Once on A3 (and once you know how to use it)... re-importing just a few thousand photos (as you claimed) is trivial to complete. It is just not a big job.

I personally think that Robert Boyer's ebooks are the single best investment you can make. They are ridiculously inexpensive and will make you understand how to really use A3.